The mixed-function oxidase system includes cytochromes P-450 that metabolize a variety of drugs and carcinogens. The multiple forms of this enzyme display broad, overlapping substrate specificity. The type and amount of each form varies among species and individuals. The focus of this project is the identification, characterization, and elucidation of structure-function relationships of these isoenzymes. Monoclonal antibodies (MAbs) to specific cytochromes P-450 are an essential tool in these studies. In particular, a Sepharose-based immunoadsorbent has been prepared with a MAb to the major cytochrome P-450 induced in the livers of rats treated with 3-methylcholanthrene. The resin strongly adsorbs from microsomes some cytochrome P-450 which appears as two polypeptides of MW 50,000 and 52,000. The MAb interaction with the former species relatively acid labile. The immunoadsorbent also binds with comparable affinity a lesser amount of a polypeptide of MW 48,000 from liver microsomes of untreated and phenobarbital-treated rats. These results demonstrate the utility of the MAb approach to analysis and preparation of various cytochromes P-450.